When one caught by individual is slander (9)
I believe the answer is:
aspersion
'slander' is the definition.
('aspersion' can be a synonym of 'slander')
'when one caught by individual' is the wordplay.
'when' becomes 'as'.
'one' becomes 'i' (Roman numeral).
'caught by' means one lot of letters goes inside another.
'individual' becomes 'person' ('person' can be a synonym of 'individual').
'i' put within 'person' is 'persion'.
'as'+'persion'='ASPERSION'
'is' is the link.
(Other definitions for aspersion that I've seen before include "Cast this slur" , "Defamatory suggestion" , "Attack on reputation" , "Negative comment" , "No praises given to the act of defaming 99)" .)